A lease is a rental agreement. So if you leased a car 35 mos., and decide to buy it out at the end, as the lessor, I would sell you the car for whatever it's worth full retail, used. Your lease payments do not reduce that amount. I'm not doing any favors, I'm in the business to maximize my profits. I can't give you my asset for less than I can get on the open market, at a given point in time.
In other words, say the car is worth $16,000 wholesale, and $24,000 retail. Your buyout is $24,000.
I have never seen a lease that did not have a pre-set Residual value, that is the buyout at the end. Yes typically there are penalties for early buy out but at lease end the residual should be what you pay. I don't know how you would calculate lease payments without a pre set residual.